The present invention broadly relates to measuring probes and, more specifically, to a new and improved construction of a measuring probe for the potentiometric determination of ion concentrations, a process for its manufacture and its application.
Generally speaking, the measuring probe of the present invention is intended for the potentiometric determination of ion concentrations and comprises a reference element, an electrolyte and a housing formed of electrically insulating material provided with at least one hollow space for containing the reference element and the electrolyte and also having an aperture via which the electrolyte may be brought into contact with a sample solution located outside the housing. The measuring probe also comprises an ion-permeable polymer at least partially filling the hollow space. Pores of the ion-permeable polymer contain the electrolyte.
Numerous measuring probes of many different kinds for the potentiometric determination of ion concentrations and/or ion activities are known in the prior art. Generally, these comprise a diaphragm, which may be in the form of a porous rod of ceramic material, via which a reference and/or a bridge electrolyte contained within the probe and usually in liquid form may be brought into contact with the sample solution being measured. However, if measuring probes of this kind are used, in particular for monitoring and/or controlling microbiological processes, contamination of the diaphragm may lead to vitiation of the measuring results, producing serious errors of up to 60 mV. It is known that the majority of erroneous measuring results are due to this cause.
Other measuring probes are also known in the prior art containing an electrolyte in gel form. Since the electrolyte in electrodes of this kind is already in gel form when it is introduced into the probe housing according to the prior art, it is impossible to avoid hollow spaces or voids within the housing, so that such measuring probes can in general only withstand pressures of up to 10 bars.
Further, German Patent Publication No. 3,100,302, published Dec. 10, 1981, describes a measuring probe suitable for the analysis of micro-quantities of biological liquids in which an aperture in the housing leading to the interior of the probe is closed by a gel containing an aqueous solution of a neutral salt, the space within the housing being filled entirely or partially with this salt solution and/or the gel which contains such salt solution and has been formed in situ. The gel used for this purpose has a comparatively low viscosity and a comparatively high water-permeability, with the consequence that, on the one hand, the probe can only be used under constant and non-critical conditions (constant temperature, for example 37.degree. C., and no pressure), so that it is not suitable for industrial applications such as process monitoring and/or process control, while, on the other hand, special arrangements are required to counter the impoverishment of the gel in neutral salt during long-term operation owing to its high water-permeability, for example by providing a feed tube via which fresh neutral salt solution can be added under pressure from a reservoir.
In an other electrode construction such as known, for example, from Japanese Patent No. 54-39 155, published Nov. 26, 1979, a solid electrolyte like, for instance, potassium chloride is mixed with a liquid curable resin and the mixture is cured in situ. The desired electrolytic conductivity is obtained by soaking the cured mixture with water from a saturated potassium chloride solution.
Furthermore, from the "HANDBOOK OF PLASTICS AND ELASTOMERS", Editor Ch. A. Harper, McGraw-Hill Book Company, 1975, pages 8-18 through 8-27, different types of fillers such as finely divided silica are known to modify viscosity, pot life, exothermicity, cure shrinkage, density, heat resistance, thermal conductivity, thermal coefficient of expansion, strength, machinability, hardness, wear resistance, electrical properties, chemical and solvent resistance, friction characteristics, thermal shock resistance, adhesion and color in epoxy and polyester resins.
A reference electrode such as known, for example, from U.S. Pat. No. 4,002,547, granted Jan. 11, 1977, contains a saturated potassium chloride solution in a container made of a hydrophobic polymer with incorporated salt particles and a metal oxide filler.
A further drawback of measuring probes of this kind is that, after long periods of use, aging phenomena may occur, resulting in potential drifts which can have an adverse effect on measuring accuracy. It is difficult to monitor the aging effects on measuring probes of this kind without carrying out numerous and complicated measurements.
Another problem with measuring probes of the aforesaid kind which has not yet been solved in the prior art consists in the fact that the reference potential is often unstable over long periods of use, and generally falls off considerably, particularly when the probe is used under pressures in excess of 10 bars or under fluctuating pressure conditions, in the presence of ultrasonic vibrations or for measuring heavily contaminated sample solutions.